Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstrasse 4+6, 53115, Bonn, Germany.
Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470, Muelheim an der Ruhr, Germany.
Angew Chem Int Ed Engl. 2017 Dec 18;56(51):16395-16398. doi: 10.1002/anie.201708305. Epub 2017 Nov 22.
Azolium cations are widely employed in organocatalysis to catalyse highly valuable synthetic processes in the presence of a base. These reactions are called "N-heterocyclic carbene catalysis", based on the assumption that they are initiated by the formation of a free carbene through deprotonation, which can then react with the substrates and thereby affect their reactivity to obtain the desired products. However, we herein provide evidence that an electrophilic aromatic substitution mechanism is energetically more favourable, in which the azolium cation reacts directly with the substrate, avoiding the formation of the free carbene in solution.
氮杂环卡宾阳离子被广泛应用于有机催化，在碱的存在下催化高价值的合成过程。这些反应被称为“N-杂环卡宾催化”，其依据是假定通过去质子化形成游离卡宾来引发反应，然后游离卡宾可以与底物反应，从而影响其反应性，以获得所需的产物。然而，我们在此提供的证据表明，亲电芳香取代机制在能量上更为有利，其中氮杂环卡宾阳离子直接与底物反应，避免了游离卡宾在溶液中的形成。
